Distending pressure of lung is determined by the transpulmonary pressure (PTP. Shown in this figure are axial (left) and sagittal views (right) of the thoracic cage representing the changes that occur as a consequence of prone positioning compared with supine positioning. The types of V'A/Q' mismatch causing impaired gas exchange vary characteristically with different lung diseases. Physiology of prone positioning in acute respiratory distress syndrome. Calculations of alveolar-arterial oxygen tension difference, venous admixture and wasted ventilation provide quantitative estimates of the effect of V'A/Q' mismatch on gas exchange. ![]() Because of the respiratory drive to maintain a normal arterial PCO2, the most frequent result of wasted ventilation is increased minute ventilation and work of breathing, not hypercapnia. Gas exchanging units with little or no blood flow (high V'A/Q' regions) result in alveolar dead space and increased wasted ventilation, i.e. In contrast to other causes, hypoxaemia due to shunt responds poorly to supplemental oxygen. Diffusion limitation, hypoventilation and low inspired PO2 cause hypoxaemia, even in the absence of V'A/Q' mismatch. Shunt and low V'A/Q' regions are two examples of V'A/Q' mismatch and are the most frequent causes of hypoxaemia. ![]() For each gas exchanging unit, the alveolar and effluent blood partial pressures of oxygen and carbon dioxide (PO2 and PCO2) are determined by the ratio of alveolar ventilation to blood flow (V'A/Q') for each unit. ![]() This review provides an overview of the relationship between ventilation/perfusion ratios and gas exchange in the lung, emphasising basic concepts and relating them to clinical scenarios.
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